1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus.
2. Description of the Related Art
Up to now, in a sheet processing apparatus connected to an image forming apparatus such as a copying machine or a printer, bookbinding processing including binding of a plurality of sheets each having an image formed thereon is performed. In the bookbinding processing, for example, a staple or glue is used. The bookbinding processing using the staple includes corner or side stitching for stitching a sheet bundle at a corner thereof with a staple, and saddle stitching for stitching sheets at a center thereof with a staple to be folded in half at a stapling position. The bookbinding processing using the glue includes case binding for applying a glue onto a back portion of a sheet bundle to adhere to a cover sheet having a size substantially twice as large as that of the sheet bundle so as to cover the sheet bundle with the cover sheet, and spine tape binding for gluing a band-like tape on the back portion of the sheet bundle.
In addition, there is another bookbinding processing such as ring binding in which punch holes are formed at an edge of each sheet having an image formed thereon, and the sheets are sequentially stacked to be formed into a sheet bundle, to thereby attach a ring-type binder to the punch holes of the formed sheet bundle (see Japanese Patent Application Laid-Open No. 2005-138549). The sheet bundle subjected to the ring binding is advantageous in its good appearance and ease of handling as a document. Accordingly, many users prefer the ring binding.
Incidentally, in a binding processor disclosed in Japanese Patent Application Laid-Open No. 2005-138549, in a case of performing the ring binding, first, each sheet received from an image forming apparatus is subjected to a process for forming punch holes at the edge of each sheet. Then, such bookbinding processing is executed that the ring-type binder is attached to the punch holes of the sheet bundle having a plurality of sheets that are subjected to the above-mentioned process and stacked therein.
However, such an attachment process depends on an accuracy of a relative position of the hole to be formed in each sheet, an alignment performance of the sheets to be stacked after being punched, and the like. If the accuracy of the relative position of the hole formed in each sheet is low or if the alignment performance of the sheets to be stacked after being punched is not excellent, the sheet bundle having sheets stacked therein and the hole of the sheet bundle are in a state as illustrated in FIG. 20, for example. In FIG. 20, both the accuracy of positions of punch holes Hd and the alignment performance of a sheet bundle Pa are not excellent. Accordingly, the positions of the punch holes Hd become uneven between an uppermost sheet Pa1 and a lowermost sheet Pa2 of the sheet bundle Pa. As a result, it is difficult to attach a binder Bd through the entire hole of the sheet bundle Pa.
Even when the accuracy of the relative position of the hole formed in each sheet is excellent and the alignment performance of the sheets to be stacked after being punched is also excellent, in the ring bookbinding, there arises another problem as illustrated in FIG. 21. As illustrated in FIG. 21, when a plurality of sheets P are stacked, the sheets P have a total thickness corresponding to a thickness of the sheet bundle Pa. For this reason, when the ring-type binder Bd is caused to pass through the hole of the sheet bundle Pa, at each position of a hole inner portion “i” of the uppermost sheet, a hole inner portion “iii” of the lowermost sheet, and a hole inner portion “ii” of the sheet positioned near a center of the sheet bundle Pa, a distance between the binder Bd and the sheet bundle Pa becomes shorter. Accordingly, when a relative positional relation between the sheet bundle Pa and the binder Bd to be attached deviates only by a small extent, the binder Bd may be brought into contact with the sheet bundle Pa, to be deformed, for example, and the binder Bd cannot be passed through the punch hole Hd, whereby the binder cannot be attached to the sheet bundle Pa in some cases. Further, when the binder Bd is brought into contact with the sheet bundle Pa in attaching the binder Bd, there is a fear that the sheets are abraded by the binder Bd to be damaged.
By the way, there is known a technology disclosed in Japanese Patent Application Laid-Open No. 2007-76840. In the technology disclosed in Japanese Patent Application Laid-Open No. 2007-76840, a punching device is moved so that a position of a hole to be punched by the punching device can be changed in a direction orthogonal to a binding side of sheets to be filed. Then, a controller sets a distance between the hole to be punched by the punching device and the edge on the side to be bound, based on information on a number-of-sheets counter and a shape of a clamp, and changes the position to be punched by the punching device. However, in the structure disclosed in Japanese Patent Application Laid-Open No. 2007-76840, an apparatus for moving the punching device becomes larger in scale and size, and costs thereof are high.